The present invention relates to apparatus for wet treatment of photographic films, webs of photographic paper or the like.
Commonly owned German Pat. No. 26 03 659 discloses an apparatus wherein the running web is trained over several rollers of which only one is actually shown. The illustrated roller is rotatably mounted on a driven shaft in such a way that it can move radially of the shaft when the tension of the running web (which is trained over the peripheral surface of the roller) reaches a preselected value. This causes the roller to engage a coupling which then transmits torque from the shaft to the roller so as to prevent extensive acceleration of the roller under the action of the rapidly advancing web, i.e., the roller can continue to brake the web even if the coupling is engaged. In order to normally hold the roller against rotation under the action of the running web, the apparatus of the German patent employs a leaf spring engaging the peripheral surface of the roller with a force which suffices to overcome friction between the bearing for the roller and the shaft. Thus, frictional engagement between the peripheral surface of the roller and the leaf spring must be sufficiently pronounced to invariably ensure that the shaft cannot rotate the roller by way of the bearing (when the coupling is disengaged) and that the running web is also incapable of rotating the roller, i.e., that the web is braked because it must slip relative to the peripheral surface of the roller. The leaf spring is mounted on a partition of the vessel for one or more treating liquid media through which the web is caused to advance, preferably by being mounted on rollers which define therefor a meandering path. Reference may be had, for example, to commonly owned U.S. Pat. No. 4,134,663 granted Jan. 16, 1979 to Erwin Laar et al. The provision of a meandering path for the running web normally entails the placing of a roller into the deepmost portion of a supply of liquid and two rollers above the upper level of such supply. Of course, if the web is to remain in longer-lasting contact with a particular liquid medium, the respective vessel (or the respective compartment of the vessel) can contain several rollers which alternate with rollers at a level above the liquid bath.
The main purpose of the leaf spring which is disclosed in the aforementioned German patent is to ensure that the roller cannot be set in rotary motion due to friction which prevails between one or more bearings for the roller and the peripheral surface of the driven shaft, and especially that the roller cannot rotate at a peripheral speed which is higher than the speed at which the running web is pulled or otherwise moved along its path. As a rule, torque which is transmitted in the absence of the leaf spring is not very pronounced but the speed of the running web can be quite high so that, in the absence of braking action upon the roller, the trailing portion of the running web (such trailing portion can be several meters long) is not subjected to requisite tension; on the contrary, the rollers in the developing machine advance the trailing portion of the web at a progressively increasing speed so that the trailing portion is not guided with a requisite degree of accuracy. In fact, the acceleration of several loops of the running web at the trailing end thereof can be so pronounced that the web can be caught below the advancing band (which pulls the web through the developing machine) or can cause other types of disturbances. The leaf spring which brakes the roller in the apparatus of the aforementioned German patent becomes ineffective when the coupling between the roller and the driven shaft is engaged but such spring brakes the roller under all other circumstances. This ensures that the trailing portion of a long or very long web is properly guided and adequately braked irrespective of the speed of the web.
The just discussed leaf spring operates quite satisfactorily insofar as its braking action is concerned. However, such leaf spring also exhibits certain drawbacks. First of all, the spring must be made of a high quality steel or an equivalent expensive alloy which contributes significantly to the cost of the apparatus, especially if the apparatus employs a large number of rollers each of which is braked by a discrete leaf spring. Furthermore, the leaf spring causes pronounced wear upon the peripheral surface of the roller. Still further, the leaf spring does not allow for training of the running web over several coaxial rollers except if it is installed in such position that it engages the peripheral surface in a region which is invariably remote from the path of the running web.